StartSpanFromContext cannot be used with FollowFromRef

[nbaztec]

Use Case

Given a parent context, using the following code always results in ChildOfRef:

span, ctx := opentracing.StartSpanFromContext(parentCtx, "foo")

I'd however wish to start a FollowsFromRef from the parent context.

Problem

Using an option doesn't work since the ChildOf() option is appended last and hence takes precedence. Moreover the FollowsFrom() option requires a SpanContext which would be required to be extracted beforehand

// extra (boilerplate) work that would be needed each time 
var opts []StartSpanOption
if parentSpan := SpanFromContext(parentCtx); parentSpan != nil {
	opts = append(opts, FollowsFrom(parentSpan.Context()))
}

// doesn't work either way since the `ChildOf` option takes precedence
// and adds multiple conflicting references to the span
span, ctx := opentracing.StartSpanFromContext(parentCtx, "foo", opts...)

There is no matching function that can start a FollowsFrom span from context.

Proposal

This can be solved via either of two ways:

1. Add a new function StartFollowsFromSpanFromContext with similar semantics to StartSpanFromContextWithTracer but that creates a FollowsFromRef

or,

2. Add a StartSpanOption to signal StartSpanFromContextWithTracer to use a FollowsFrom instead of ChildOf

Questions to address (if any)

• Which one of the solutions best fits the project semantics.
• If opting for 1., adding a new function for follows from, is minimal code duplication from StartSpanFromContextWithTracer okay?
• If opting for 2., adding a new option but reusing the same function StartSpanFromContext, the current documentation states that it builds a ChildOfRef - would that qualify as major version API change?